This invention relates to apparatus for the measurement of velocity of fluids, such as wind, and in particular to drag or thrust anemometers.
In the past, drag elements have been used for the measurement of both the speed and direction of fluid flow, such as wind or water. In a drag anemometer, a drag element having a spherical or similar shape is positioned in the fluid to be measured, and the flow of the fluid causes a drag force to be exerted on the drag element which is proportional to the square of the velocity of the fluid impinging on the drag element. Various methods have been used to sense both the magnitude or speed of this fluid flow and the direction of the flow. Direction is usually measured by sensing the force of the flow in two or three orthogonal directions and from this computing the true flow direction. The magnitude or speed of the flow is usually sensed by measuring the strain in or deflection of some member attached to the drag element. Gravitational or spring bias means are usually employed to offset the flow drag force on the drag element.
An example of one of these prior art devices is shown in U.S. Pat. No. 3,212,329 issued on Oct. 19, 1965 to A. Bisberg. In the type of device shown in this patent, displacement of the drag element in three axes is measured and the deflection of the drag element is opposed by the action of spring plates.
Another example of such a prior art device is shown in U.S. Pat. No. 3,264,869 issued on Aug. 9, 1966 to L. Erdely. This is a type of device that uses gravitational forces to bias the drag element and offset the drag force. The fluid speed and direction of flow is determined from a measurement of the displacement of the drag element along two orthogonal axes.
A difficulty with the prior art devices is that they attempt to calculate speed and direction of flow by measuring the displacement of the drag element and this displacement is a function of the spring or gravitational restoring forces applied to offset the fluid drag forces on the drag element. The result is that these devices have poor sensitivity in the low range of flow velocities because the dynamic forces causing displacement of the drag element are very low due to the quadratic relationship between the flow velocity and the drag force. Further, many of the prior art devices suffer from inaccuracy caused by hysteresis.
It is an object of the present invention to overcome the deficiencies of the prior art devices by balancing the flow drag force on the drag element with an electromagnetic force and measuring the current to produce this electromagnetic force. Since the electromagnetic force is proportional to the square of the current used to produce this force (with the position of the drag element remaining relatively constant), there is a direct or linear relationship between this current and the flow velocity.